1. Field of the Invention
This invention relates to a method for the production of microfine colored particles and an electrophotographic toner using the microfine colored particles obtained by this method. More particularly, this invention relates to a method for the production of microfine colored particles each having a coloring agent uniformly dispersed therein, enjoying a modified particle surface, and promising to find utility as a coloring agent in toners, coating materials, inks, molded articles of resin, etc. and an electrophotographic toner produced by using the microfine colored particles and enabled, when used as a toner in a printing device such as a laser printer or a liquid crystal printer, to form a clear image.
This invention relates to a method for the recovery of microfine globular particles formed mainly of resin from a suspension. To be more specific, this invention relates to a method for efficient recovery from a suspension of microfine globular particles obtained by the suspension polymerization method, for example.
2. Description of the Prior Art
The electrophotographic method comprises forming an electric latent image on a photosensitive material which is made of such a photoconductive material as selenium, zinc oxide, or cadmium sulfide, developing the latent image with a powder developer, transferring the developed image to a paper, and fixing the image on the paper.
Heretofore, the tone user for the development of the latent image in electrophotography has been generally produced by melting, mixing, and dispersing a coloring agent and other additives (such as a charge controlling agent, an offset preventing agent, and a lubricant) in a thermoplastic resin, solidifying the resultant dispersion, pulverizing the solid, classifying the produced particles, and collecting microfine colored particles of a desired particle diameter.
The method which produces a toner in accordance with the pulverizing step as described above, however, entails various drawbacks. Firstly, it necessitates numerous steps including the step of producing a resin, the step of mixing this resin with a coloring agent and other additives, the step of pulverizing the resultant solid mixture, and the step of classifying the produced particles and collecting microfine colored particles of a desired particle diameter and, therefore, requires use of as many devices as these steps. The toner which is produced by this method is expensive as a necessary consequence. Particularly, the step of classification constitutes itself an indispensable requirement for the production of a toner having particle diameters in a range optimum for the formation of an image enjoying high clarity and suffering sparingly from the phenomenon of fogging. It nevertheless has a problem of deficiency in productional efficiency and yield. Secondly, in the step of mixing, it is extremely difficult for the coloring agent and other additives to be uniformly dispersed in the resin. The toner produced by this method, therefore, suffers from inferior dispersion of the coloring agent, the charge controlling agent, etc. and consequent lack of uniformity of triboelectric characteristic among component particles and inevitable decline of resolution. In the future, these problems will gain all the more in prominence as a growing impetus is given to the trend of these toners toward particle diameter reduction which forms an indispensable requirement for the formation of images of high quality. The existing pulverizing devices are limited in ability to allow particle diameter reduction for such toners. Even if they are capable of producing a toner of a reduced particle diameter, the toner produced thereby at all still suffers from heavy ununiformity of the amount of charging due to inferior dispersion of the coloring agent and the charge controlling agent.
For the elimination of various drawbacks observed in the toner produced by the method of pulverization mentioned above, various methods for the production of a toner by the use of the emulsion polymerization technique or the suspension polymerization technique have been proposed (Japanese Patent Publications SHO 36(1961)-10,231, SHO 43(1968)-10,799, SHO 47(1972)-518,305, SHO 51(1976)-14,895, etc.). These methods invariably resort to a procedure of synthesizing a toner containing a coloring substance in one step of subjecting to emulsion or suspension polymerization a polymeric monomer incorporating therein the coloring substance such as carbon black and other additives. These method are capable of mending the drawbacks of the convention pulverizing method to a noticeable extent. Since they include absolutely no step of pulverization, they find no use for improvement of brittleness of the produced toner. Further, since the individual particles of the produced toner have a globular shape and excel in flowability, they enjoy uniformity of triboelectric characteristic. The method for the production of a toner by the technique of polymerization, however, has a problem of its own. Firstly, since such hydrophilic substances as a dispersant and a surfactant which are used in the process of polymerization are not completely removed in the step of washing and are suffered to persist in the toner particle surface, the charging property of the produced toner is susceptible of the influence of the environment. Secondly, since the toner particles produced by the technique of polymerization have a spheric shape and a very smooth surface, the toner deposited fast on the sensitive material is not easily removed and the sensitive material is consequently compelled to suffer from poor cleanability. Various methods aimed at solution of these problems have been proposed in Japanese Patent Application Disclosures SHO 61(1986)-255,354, SHO 53(1978)-17, 736, SHO 63(1988)-17,460, SHO 61(1986)-167,956, etc. These methods are impracticable because they bring about no fully satisfactory effects and lead to increases of cost.
For the solution of problems of this sort, methods for the production of an electrophotographic toner possessing a desired particle diameter have been disclosed which generally comprise emulsifying, dispersing, and polymerizing a polymeric monomer, a coloring agent, and/or a magnetic powder, and a polymerization initiator in the presence of an emulsifier thereby preparing a main resin component, solidifying the resultant polymerization solution at a temperature not exceeding the glass transition point of the main resin component, heating the particles resulting from the solidification at a temperature exceeding the glass transition point of the main resin component thereby effecting thorough dissolution of the particles, and classifying the resultant solid particles (Japanese Patent Application Disclosures SHO 61(1986)-167,955, SHO 61(1986)-167,956, SHO 61(1986)-167,957, and SHO 61(1986)-72,258).
In these methods, however, since the polymerization is carried out mainly in the form of emulsion polymerization, the produced polymer particles acquire a particle diameter on the order of submicrons and the coloring agent such as carbon black is not included in the polymer particles but is left standing outside the polymer particles. Even if these microfine polymer particles are occluded in larger particles in the subsequent step of solidification or thermal fusion, they cannot be uniformly dispersed therein. As a result, the produced toner suffers from lack of uniform dispersion of the coloring agent and, when used as an electrophotographic toner, entails lack of uniform electric charging or causes a draft of toner, possibly followed by the phenomenon of fogging or the defilement of the drum surface. The polymer particles obtained by polymerization in these method are so small that their particle diameters are required to be controlled as well as increased by having the polymer particles subjected to solidification in the step of solidification at a temperature not exceeding the glass transition point of the resin. The solidification necessitates use of an inorganic acid or an organic acid as a solidifying agent. This solidifying agent cannot be thoroughly removed from the toner no matter how thoroughly the toner may be washed. The residual solidifying agent has the problem of rendering the toner resistant to dependency on the environment and compelling a decline in the electric property of the toner.